1. FIELD OF THE INVENTION
This invention relates to barium titanate ceramic compositions suitable for production of reduction-reoxidation type semiconducting capacitors.
2. DESCRIPTION OF THE PRIOR ART
Conventional semiconducting capacitors include a grain boundary insulating portion obtained by diffusing Cu, Bi, Mn, etc. into the grain boundaries of crystalline semiconductor ceramics, a surface barrier layer and a reduction-reoxidation surface layer of a semiconducting ceramic.
The grain boundary insulating portions of the semiconducting capacitors are produced from BaTiO.sub.3 and SrTiO.sub.3 ceramics, and in recent years, the SrTiO.sub.3 ceramics have been mainly used because of their better temperature and bias characteristics. Capacitors of this type also have the advantage that their characteristics vary little with a change in the type of electrode used, and they are highly reliable. However, their cost is high because they are produced by a complex manufacturing process. Moreover, their breakdown voltage is low, and their capacitance per unit area cannot be made high.
With the surface barrier layer-type semiconducting capacitors, a relatively large capacitance per unit area can be obtained but they have a relatively low breakdown voltage and low insulation resistance and this limits the range of their application.
Generally, reduction-reoxidation type semiconducting ceramic capacitors have the advantage that is typical of the grain boundary insulating type and surface barrier layer type semiconducting ceramic capacitors. Namely their capacitance per unit area is much greater than conventional dielectric ceramic capacitors so they can be manufactured in a small size with a large capacitance. Advances in electronics technology have led to great demand for capacitors of reduced size and increased capacitance. However, when an attempt is made to obtain a capacitance of at least 0.7 .mu.F/cm.sup.2 by using a conventional barium titanate semiconducting ceramic composition for a reduction-reoxidation type semiconducting ceramic capacitors, the breakdown voltage or insulation resistance of the capacitors are drastically reduced. Hence, the capacitance of such capacitors can be increased only to about 0.5 .mu.F/cm.sup.2 at the highest.
Generally, a reduction-reoxidation semiconducting ceramic capacitor is made by heat-treating a ceramic composition composed of a solid solution such as BaTiO.sub.3 -La.sub.2 O.sub.3 -TiO.sub.2 with an oxide of manganese added thereto. This composition is subjectd to a reducing atmosphere to convert it into a semiconductor, then the semiconductor is heat-treated in an oxidizing atmosphere to form a dielectric re-oxidized layer on the surface of the semiconductor, and then the electrodes are applied. Accordingly, its electrical properties, such as capacitance, insulating resistance, breakdown voltage and temperature characteristics, depend greatly upon the re-oxidized dielectric layer formed on the surface of the semiconductor ceramic. In order, therefore, to increase the capacitance per unit area of semiconductor ceramics of the same composition, the re-oxidized layer on the surface of the semiconductor ceramics should be reduced in thickness. A reduction in the thickness of the re-oxidized layer, however, results in a reduction in breakdown voltage and insulation resistance.
To reduce size and increase capacitance and simultaneously to increase breakdown voltage and insulation resistance, the following three conditions should be met simultaneously.
(1) The dielectric constant of the dielectric ceramic should be high.
(2) The ceramic should be dense structure and have a small and uniform crystal grain size,
(3) The speeds of reduction and reoxidation should be high.
With the conventional composition mentioned above, however, reduction of the crystal grain size results in a dielectric constant of as low as about 8000 to 10000. To increase the dielectric constant to more than 12000, on the other hand, requires crystal grain sizes of as large as 3 .mu.m or more. Thus, when the capacitance is increased to 0.7 .mu.F/cm.sup.2 or higher, the breakdown voltage or insulation resistance of reduction-reoxidation type semiconducting ceramic capacitors is drastically reduced so that they become useless for practical purposes.